1. Field of the Invention
The present invention relates to monopulse tracking radars and more particularly to a method and system including a novel monopulse tracking radar antenna and antenna positioning system for tracking targets at low elevation angles.
2. State of the Prior Art
When the elevation angle of a target being tracked by a monopulse radar is less than approximately one beamwidth, troublesome multipath return signals or target echoes are received by the tracking radar system. The receipt of multipath signals typically occurs when the tracking radar is adjacent a reflective surface such as the sea, and energy reflected from a target is received both directly from the target and from the reflective surface. As a result of the multipath signals entering the tracking radar feedhorn by way of the direct path and the reflected path, two unresolved targets at about the same range are within the radar beam at the same time. The radar system simultaneously "sees" the actual target and an image of the actual target because of the received multipath signals.
The monopulse radar system, in the presence of multipath signals angularly separated by less than approximately one beamwidth combines the multipath signals and attempts to null on the composite signal. The radar may then either oscillate about a mean position or may oscillate in what appears to be a random fashion over an angle of about one beamwidth centered about the bisector of the angle between the target and its image. The monopulse radar will frequently lose track altogether under these circumstances.
Several possible solutions to the problem of multipath signals at low target elevation angles have been explored. For example, by increasing either the aperture or the frequency of the monopulse radar system, the resolution may be improved thus permitting lower elevation angle tracking without the above discussed problems. However, this solution merely decreases the likelihood of encountering multipath problems and is difficult to apply to the existing radars.
The use of passive screens to screen out undesired multipath signals is quite impractical at sea, and in any case replaces the multipath problem with one of diffraction over the top of the screen. Other possible solutions which appear to have merit conceptually include distinguishing between the direct and reflected signals on the basis of time of arrival, doppler difference, or polarization. Separation of multipath signals on the basis of time of arrival or doppler difference is very difficult in practice in that these differences are so small that either a very wide bandwidth or unachievable doppler resolution would be required. Similarly, the separation of multipath signals on the basis of polarization can be shown to be impractical since, at the frequencies and elevations of interest, the polarization of the return signal is not changed on reflection from the sea surface.